Refactorings after shutdown of legacy serlo.org

[kulla]

• [x] https://github.com/serlo/api.serlo.org/issues/133
• [x] Migrate all revisions to edtr-io so that we can remove code for the markdown editor
• [ ] Create dump of current version
• [x] Drop related content from DB
• [x] Delete MySQL table license
• [x] Delete table ad (horizon) and blog
• [ ] Delete taxonomyTypes besides topic and topicFolder
• [x] delete table comment_vote
• [ ] remove all comments/threads on pages

[CodingDive]

Cross linking the issue on db migrations https://github.com/serlo/documentation/issues/4

[elbotho]

is horizon still in there?

[kulla]

is horizon still in there?

Yep, it's ad